SOLVED. YA Mystery. Teens solving secret involving white haired girl, while running around in secret tunnels in big city. Read early 2010s

[[deleted user]]

MC was a nerdy girl with dark hair. She's an only child, her parents are scholars whose life is funded by some relative as long as they study. Their house is filled with books.
I don't think MC was the social kind, she made friends along the way. The city was maybe NY or London, definitely big. They spent a lot of time in some secret tunnels or sewers, trying to help the white haired girl.
Cover featured at least the white haired girl, maybe the MC too. It was drawn digitally.
I've read it in Germany in the early 2010s but it was definitely a translation.

[Becca]

Inside the Shadow City by Kirsten Miller, first book in the Kiki Strike series? The German edition is Die Schattenstadt

[Kris]

OP moved this thread to a Solved folder. Their account is deleted.